Fine grain rare earth alloy cast strip, preparation method thereof, and a rotary cooling roll device

ABSTRACT

An alloy cast strip includes grains having R 2 Fe 14 B-type compound as main phase, where R denotes a rare earth element, Fe denotes iron, and B denotes boron. The grains include non-columnar grains having an aspect ratio in a range from 0.3 to 2 and columnar grains having an aspect ratio equal to or larger than 3. A ratio of an area of the non-columnar grains to a total area of the grains is equal to or larger than 60% and a ratio of a number of the non-columnar grains to a total number of the grains is equal to or larger than 75%. A ratio of an area of the columnar grains to the total area of the grains is equal to or smaller than 15% and a ratio of a number of the columnar grains to the total number of the grains is equal to or smaller than 10%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2017/111025, filed Nov. 15, 2017, which claims priority to ChineseApplication Nos. 201611244386.6, 201611244721.2, and 201611245318.1, allfiled Dec. 29, 2016.

TECHNICAL FIELD

The present disclosure relates to rare earth alloy cast strip and itspreparation method, specifically relates to alloy cast strip for finegrain rare earth sintered magnets, its preparation method and a rotarycooling roll device.

BACKGROUND

The trend of industrial automation and the expansion of the demand forclean energy represented by electric vehicles have provided new marketopportunities for rare earth permanent magnets, but at the same timethey have increased the requirements for magnet performance. Forexample, Nd—Fe—B magnets for electric vehicles generally need to containat least 5 to 6% by mass of a heavy rare earth element such as Dy toimprove the high temperature resistance of the magnet. However, due tothe risk management of heavy rare earth elements such as Dy and thecontinuous pursuit of higher performance of magnets, reducing the amountof heavy rare earth has become an important issue for Nd—Fe—B magnettechnology while improving or maintaining the existing performanceindicators.

Recent trends in Nd—Fe—B magnet technology show that there are two mainroutes for reducing the amount of heavy rare earth and furtherincreasing the coercivity of the magnet to improve its thermalstability: 1 heavy rare earth (such as Dy, Tb, etc.) element boundarydiffusion technology (GBD); 2 magnet grain refinement technology. Thegrain boundary diffusion technique (GBD) has enabled the magnet toreduce the heavy rare earth content of about 2 to 3% by mass whilemaintaining or slightly improving the existing performance. It isexpected that the coercive force can be remarkably improved by furtherrefining to an average particle diameter of no more than 3 μm on thebasis of the existing average magnet grain size of about 6 to 10 μm. Onthe basis of the existing mass production technology, the amount ofheavy rare earth elements in the mass ratio of 1 to 2% can be furtherreduced, and it is expected that the rare earth permanent magnets havinglow or heavy rare earth elements and satisfying the performancerequirements of electric vehicles can be finally obtained. Therefore,the grain refinement technology has important practical applicationvalue for various types of rare earth permanent magnets represented byNd—Fe—B.

As the first process of industrial production of modern Nd—Fe—B magnets,the preparation of alloy strips has laid a foundation for the entiremanufacturing process of magnets. The quality of alloy strips has acritical impact on the performance of the final magnets.

It has been reported in literature that spacing of Nd-rich phases ofstrip casting flakes is even and uniform which is of positivesignificance for the current mass production of magnets. However, themicrostructure of the prepared strips is essentially a columnar crystalwith the particle on surface of the cooling roll as a heterogeneousnucleation center and radially growing along the temperature gradientdirection, and the improvement is mainly to reduce spacing ofrare-earth-rich phases of the columnar grains distributed along thetemperature gradient direction. The spacing of plate crystalrare-earth-rich phases on free surface side is usually larger than thaton the surface side of the roll, and the overall spacing deviation isgreater than 3 It is not conducive to the uniformity of powder duringits preparation. At the same time, the spacing of rare-earth-rich phaseof such alloy cast strips is too large, which is not conducive to grainrefinement. When the powder with a particle size of about 3˜5 μm isprepared, the rare-earth-rich phase loss is large. With the demand forgrain refinement, the particle size of the jet mill powder is furtherreduced, and the effective utilization rate of the rare earth is furtherreduced, which is not conducive to improving the coercive force of thefinal magnet. At the same time, the growth mode along the direction ofthe temperature gradient easily leads to macroscopic segregation of thealloy composition in this direction, which may increase the unevennessof the microscopic magnetocrystalline anisotropy in the local region ofthe final magnet and reduce the coercive force of the magnet.

SUMMARY OF THE DISCLOSURE

In view of the above problems, the present disclosure provides a finegrain rare earth alloy cast strip, a preparation method thereof, and arotary cooling roll device used in the preparation process. The innergrains of the alloy cast strip prepared according to the presentdisclosure are fine and uniform, and the spacing of the rare-earth-richphases is small. When the sintered rare earth magnet is prepared byusing the alloy cast strip, the utilization ratio of the rare earth andthe uniformity of the powder can be improved, and the coercive force ofthe final magnet can be improved.

One purpose of this disclosure is to provide an alloy cast strip for afine-grain rare earth sintered magnet having a roll surface and a freesurface, characterized in that the alloy cast strip comprises grainswith R₂Fe₁₄B-type compound as their main phase, and the grains includenon-columnar grains and columnar grains along a temperature gradientcross section. In some embodiments, non-columnar grains having an aspectratio of 0.3 to 2 account for

60% of the area of the grains and account for

75% of the number of grains; and columnar grains with an aspect ratio

3 account for ≤15% of the area of the grains and account for ≤10% of thenumber of grains.

In another aspect, this disclosure provides an alloy cast stripcomprising R₂Fe₁₄B-type main phase, in-grain rare-earth-rich phaseembedded in a grain and boundary rare-earth-rich phase distributed onthe boundary of the grains, wherein the spacing of the in-grainrare-earth-rich phases is 0.5-3.5 μm.

Furthermore, the alloy cast strip comprises a rare earth element R, anadditive element T, iron Fe and boron B; wherein the R is one or more ofLa, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Sc, Y; the T is one or more of Co, Ni,Cu, Mn, Cr, Ga, V, Ti, Al, Zr, Nb, Sn.

In some embodiments of the present disclosure, the mass ratio of B inthe alloy cast strip is from 0.85% to 1.1%.

Furthermore, the equivalent circle diameter of the grains is 2.5 to 65μm in a section along a temperature gradient direction.

Furthermore, the crystal grains having an equivalent circle diameter of10 to 50 μm account for ≥80% of the area of the crystal grains. Thecrystal grains having an equivalent circle diameter of 15 to 45 μmaccount for ≥50% of the number of the crystal grains.

Furthermore, in the cross section of the temperature gradient direction,the average equivalent circle diameter of the grains in the range of 100μm near the roll surface is 6 to 25 μm; the average equivalent circlediameter of the grains in the range of 100 μm near the free surface is35 to 65 μm.

Furthermore, the area of grains having a heterogeneous nucleation centeroccupies ≤5% of the area of the alloy cast strip.

Furthermore, the grains are not in a through-grown state from the rollsurface to the free surface.

Furthermore, the rare-earth-rich phase is not in a through-grown statefrom the roll surface to the free surface.

Furthermore, the grain boundaries have a rare-earth-rich phasedistributed in an irregularly closed configuration along a temperaturegradient direction cross section.

In some embodiments of this disclosure, the grain has a primary crystalaxis and a secondary crystal axis; wherein the secondary crystal axisgrows based on the primary crystal axis. The width L1 of the primarycrystal axis at the minor axis direction is 1.5 to 3.5 μm; the width L2of the secondary crystal axis at the minor axis direction is 0.5 to 2μm.

Furthermore, the rare-earth-rich phase between the secondary crystalaxes is distributed in a short straight line or a discontinuous dottedline.

The disclosure also provides a method for preparing the above alloy caststrip for fine grain rare earth sintered magnet, comprising thefollowing steps:

The rust-removed alloy raw material is placed in a crucible, and thecrucible is placed in the induction melting furnace; the impurity gasadsorbed by the alloy raw material is excluded; the power of theinduction melting furnace is controlled, and the alloy raw material iscompletely melted by the cyclic heat treatment before the surfacetemperature of the melt is raised to 1300° C.; after the alloy rawmaterial is melted, the power of the induction melting furnace isadjusted to stabilize the surface temperature of the melt at anytemperature in the range of 1400° C. to 1500° C.; and the surface linearvelocity of the rotary cooling roll device is controlled to be 1.5 to2.25 m/s, and the melt is uniformly and smoothly arranged on the surfaceof the rotary cooling roll device for casting cooling to obtain an alloycast strip.

In some embodiments of this disclosure, a metal having a higher meltingpoint in the alloy raw material is placed at the bottom of the crucible,and a metal having a lower melting point is placed on the upper portionof the crucible.

In some embodiments of this disclosure, in the induction meltingfurnace, the impurity gas adsorbed by the alloy raw material is removedby vacuuming-filling with an argon gas; the argon gas is high purityargon with its volume fraction

99.99%.

In some embodiments of this disclosure, the ten point average roughnessof the rotary cooling roll device surface ranges from 1 to 10 μm.

In some embodiments of this disclosure, in the casting cooling process,the ratio of the casting speed q of the melt to the cooling water flowrate Q in the rotary cooling roll device is q/Q=0.05 to 0.1.

In some embodiments of this disclosure, during casting cooling, thedifference between average temperature of alloy cast strip on thehighest point of rotary cooling roll and melting point of alloy mainphase ranges from 300˜450° C.

This disclosure also provides a rotary cooling roll device for use inthe above mentioned method, comprising an inlet pipe, a water inletsleeve, an outlet pipe, a water outlet sleeve, an internal heat exchangepassage, a rotary cooling roll outer casing, wherein the inner heatexchange flow passage is nested inside the rotary cooling roll device,the rotary cooling roll outer casing is an inner spiral structureprepared from a copper-chromium alloy, and forms a spiral water channelwith the inner heat exchange flow passage; the rotary cooling a frontend cover and a rear end cover are fixed on both sides of the roll outercasing, the front end cover is provided with a water inlet hole; theinner heat exchange flow path is a hollow structure, and a heatconducting sheet perpendicular to the front end cover is embedded; onthe internal heat exchange, a water inlet hole is disposed on a side ofthe front end cover, and a water outlet hole is disposed on a side ofthe front end cover; the water inlet pipe and the water outlet pipe aredisposed on the rotary joint, and both ends of the water inlet sleeveare connected to the rotary joint and the water inlet hole of the innerheat exchange passage respectively, the two ends of the water outletsleeve are respectively connected with the water inlet of the rotaryjoint and the front end cover, and the inner diameter of the wateroutlet sleeve is larger than the inlet water sleeve outer diameter.

Furthermore, the number of the heat conducting sheets is plural.

Furthermore, the water outlet sleeve is fixedly connected to the frontend cover through a sealing sleeve. On the inner heat exchange flowpassages, one or more water outlet holes are disposed adjacent to therear end cover side.

The alloy cast strip prepared by the method of the disclosure has agrain aspect ratio in the range of 0.3 to 4 in the alloy cast stripalong the temperature gradient direction section, and the equivalentcircle diameter of the crystal grain is in the range of 2.5-65 μm. Thespacing of in-grain rare-earth-rich phases is in the range of 0.5 to 3.5μm. The distribution of the rare-earth-rich phase is less affected bythe temperature gradient, the distribution is more uniform, and thedifference between the roll surface side and the free surface side issmaller. For magnet prepared by the alloy cast strip which is chemicallycrushed and mechanically crushed, the obtained powder has a more uniformparticle size and higher rare-earth-rich phase adhesion rate. The growthmode of the grains in the alloy cast strip is different from the radialgrowth in the conventional technologies (that is, growth along thetemperature gradient), which is advantageous for suppressing themacrosegregation of the composition of the alloy cast strip andimproving the coercive force of the final magnet product.

In the rotary cooling roll device of the present disclosure, a spiralwater passage can be formed between the inner heat exchange passage andthe rotary cooling roll outer casing. Moreover, the radialheat-conducting sheet embedded in the inner heat exchange flow channelcan increase the contact area of the cooling water and the solidheat-dissipating component, improve the heat exchange capability, andthereby improve the overall cooling capacity of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a polarizing microscope photograph of an alloy cast strip ofthe present disclosure.

FIG. 2 is a polarizing microscope photograph of an existing alloy caststrip.

FIG. 3 is a schematic diagram showing the definition of the aspect ratioof the crystal grains.

FIG. 4 is a schematic view showing the growth of crystal grains along atemperature gradient in an existing alloy cast strip.

FIG. 5 is a schematic diagram showing the measurement of the spacing ofthe rare-earth-rich phases.

FIG. 6 is a schematic flow chart of a method for preparing an alloy caststrip according to an embodiment of the present disclosure.

FIG. 7a is a schematic structural view of a rotary cooling roll devicein accordance with an embodiment of the present disclosure.

FIG. 7b is an axial cross-sectional view of the inner wall of the innerheat exchange passage in the rotary cooling roll device.

FIG. 8 is an optical micrograph (600-time magnification) of a Nd—Fe—Balloy cast strip having a layered structure.

FIG. 9 is a polarizing microscope photograph of the alloy cast strip ofExample 1 and the identification of the crystal grains (800 timesmagnification).

FIG. 10 is a scanning electron microscope (SEM) photograph of alloy caststrip in Example 1.

FIG. 11 is polarizing microscope photograph of the alloy cast strip andthe identification of the crystal grains of Comparative Example 1.

FIG. 12 is a scanning electron microscope (SEM) photograph of alloy caststrip in Comparative Example 1.

FIG. 13 is a polarizing microscope photograph (800× magnification) ofthe alloy cast strip of Example 2.

FIG. 14a is a scanning electron microscope photograph (600×magnification) obtained in situ in the observation area of FIG. 13.

FIG. 14b is an enlarged photograph (4000× magnification) of a partialarea in the lower middle of FIG. 14 a.

FIG. 15 is an scanning electron microscope photograph of alloy caststrip in embodiment 3.

FIG. 16 is a polarizing microscope photograph of the alloy cast strip inExample 3.

FIG. 17 is a scanning electron microscope (SEM) photograph (1000-foldmagnification) of alloy cast strip of Comparative Example 2.

FIG. 18 is a scanning electron microscope (SEM) photograph (1000-foldmagnification) of alloy cast strip of Comparative Example 3.

FIG. 19 is a photograph showing the grain identification of FIG. 16.

FIG. 20 is a histogram showing the distribution of the number of grainswith the aspect ratio and the equivalent circle diameter of the alloycast strips prepared in Example 1, Example 3, and Comparative Example 1.

FIG. 21 is a graph showing the cumulative distribution of the grain areawith the aspect ratio of the crystal grains and the equivalent circlediameter of the alloy cast strips prepared in Example 1, Example 3, andComparative Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present disclosure will be described in moredetail in conjunction with the accompanying drawings and examples inorder to provide a better understanding of the embodiments of thedisclosure and its advantages. However, the specific embodiments andexamples described below are illustrative only rather than limiting thedisclosure.

As shown in FIG. 8, part of the Nd—Fe—B alloy cast strip can show adistinct layered structure during a preparation process.

In FIG. 8, the lower part is the roll surface, and a thin layer of finechilled crystal appears. The upper part is a free surface, and theNd-rich phase has a clear growth trend along the temperature gradientdirection, but the grain boundary is difficult to distinguish by theordinary light microscope and scanning electron microscope. The grainboundaries in the central region are clearly visible, and the innerNd-rich phase is smaller and the spacing is smaller than that in theupper free surface region. Among them, some of the grain Nd-rich phasedistribution traces are inconsistent with the temperature gradientdirection, even perpendicular to the temperature gradient direction.

The melt solidification process in the middle region is different fromthe roll surface and the free surface, and may be a special transitionstate between the two. The present disclosure aims to promote theformation of the intermediate layer, and at the same time suppress theratio of chilled crystal on the roll surface and gradient growth layeron the free surface, and prepare an alloy cast strip for the fine grainrare earth sintered magnet. The preparation method is shown in FIG. 6.

The preparation process of the alloy cast strip mainly includes thesteps of alloy melting and casting cooling:

(A) Alloy Melting

For this step, attention needs to be paid to the following two points.

(1) The Impurity Gas Adsorbed by the Raw Material is SufficientlyExcluded.

In an embodiment of the disclosure, an alloy is smelted using aninduction melting furnace. First, the alloy raw material is subjected torust removal treatment, and the raw material is placed in a crucibleaccording to the formulation of the alloy cast strip, and the crucibleis placed in an induction melting furnace. In the present disclosure,the Fe having the largest proportion of the alloy and having a highermelting point is usually placed at the bottom of the crucible, and therare earth and rare earth alloy having a relatively low melting pointare placed on the upper portion of the crucible.

Close the induction melting furnace l/d and evacuate to 10⁻²˜10⁻³ Pa.The vacuum is continued under low power and slow heating. Afterlow-power heating for 3 to 5 minutes, the power is appropriatelyincreased, and the operation is repeated until the internal material ofthe crucible emits a red luster due to an increase in temperature. Then,the vacuum valve is closed to charge the induction melting furnace withhigh-purity (

99.99%) argon gas until the gas pressure in the furnace reaches 40-50kPa for 0.5 to 1 minute. Reopen vacuum valve to vacuum to 10⁻² Pa andcharged with argon to 40 kPa again. In this stage, the heating power,heating time and temperature of the raw materials in the crucible can beadjusted according to the actual working conditions, without strictrequirements, and can be repeated many times. The purpose of thisoperation is to completely exclude the impurity gases adsorbed by theraw materials, especially oxygen.

(2). Carry Out Low Temperature Cyclic Overheat Treatment, High PowerTemperature Refining, Purification of the Melt.

After the impurity gas is sufficiently removed, the power of theinduction melting furnace is gradually increased until the alloy beginsto melt, thereby forming a melt. The melt surface temperature measuredusing a dual colorimetric infrared thermometer is within a range of1050° C.˜1200° C., but high-melting raw materials such as metallic ironare not completely melted. Adopt high power and low power oscillationcontrol, and perform cyclic heat treatment under protective atmosphereto make the melt fluctuate warm up in small temperature (50˜100)° C.range. Ensure that the alloy raw materials are completely melted beforewarming up to 1300° C.

The cyclic overheat treatment process in the present disclosure is asfollows: for example, the alloy melt may begin to melt at 1150° C., buthigh melting point metals such as iron are not completely melted andstill exist as bulk metals. Keep heating power constant or increaseheating power to raise the melt temperature to 1200° C. After 30 to 60seconds, reduce the heating power or stop the heating to lower the melttemperature to 1100° C. and remain at this temperature for 30 to 60seconds. Then increase the heating power to raise the melt temperatureto 1250° C. and remain for 30 to 60 seconds, then reduce the heatingpower again and wait for the melt temperature to drop back to 1200° C.Then increase the heating power again and wait for the melt temperatureto rise to 1300.° C. And keep it for 30 to 60 seconds. During the cyclicoverheat treatment process, the bulk metal iron gradually melts anddisappears, but the internal composition of the melt fluctuates greatly,and at the same time, with the melting or precipitation of γ-Fe andother unknown alloy particles, the inherent heterogeneous nucleationcenter melt can be reduced or passivated to some extent, which isconducive to purify melt and reduce the heterogeneous nucleation rateduring melt solidification.

After the alloy raw material is melted to obtain a melt, the power ofthe induction melting furnace is increased, and the stirring effect ofthe induced electromagnetic wave on the melt is enhanced. When the meltsurface temperature rises to 1400° C., the temperature increasing rateis reduced by adjusting the power, and the final melt temperature isstabilized at a temperature in the range of 1400.° C.˜1500° C. (“stable”refers to a temperature fluctuation ≤30° C. within one minute). Duringthis operation, the oxide in the melt mostly adheres to the cruciblewall as a dross, and a small amount floats on the surface of the meltwithout affecting the progress of the casting process. At this point,the melt reaches the casting state.

The purpose of this step is to optimize the melt state, purify the melt,and to make the internal temperature of the melt uniform, and towithstand greater subcooling in the subsequent casting cooling step withthe necessary conditions for thermodynamic deep subcooling. Controllingthe melt temperature to be not lower than 1400° C. can reduce the numberof large atomic groups in the melt, thereby reducing the size ofinterior critical nucleus of the melt during the non-equilibriumsolidification. At the same time, during deep subcooling, it isbeneficial to reduce the activation energy in the process of meltnucleation and increase the probability of homogeneous nucleation. Insummary, due to the lack of a sufficient nucleation center in purifiedinterior of the melt, the nucleation rate of the melt on the surfaceside of the roll is suppressed, and thereby the formation of the chilledcrystal region is suppressed. At the same time, it is beneficial to thesuper deep cooling of the melt, which increases the probability ofhomogeneous nucleation in the melt.

(B) Casting Cooling

The casting cooling process may include: quasi-static heat exchangebetween the melt and the cooling roll; and unbalanced rapid transport ofthe heat of the cooling roll by the water body. The heat transfercoefficients of copper and water are 401 W/(m·K) and 0.5 W/(m·K),respectively. In order to remove the heat from the surface of the chillroll in time, the casting flow rate and the cooling water flow rate needto be matched. Moreover, the water channel design of the chill roll isalso critical because the heat exchange efficiency of the cooling rollouter casing and the water body directly affects the cooling capacity ofthe equipment.

FIGS. 7a and 7b show a rotary cooling roll device according to anembodiment of the present disclosure. As shown in FIG. 7a , the rotarycooling roll device comprises: an inlet pipe 1, a rotary joint 2, anoutlet pipe 3, a water outlet sleeve 4, a water inlet sleeve 5, asealing sleeve 6, a front end casing 7, an internal heat exchangepassage 8, and a heat conducting sheet 8.1, a cooling roll outer casing9 and the rear end cover 10. Among them, the rotary joint 2 can realizethe relative rotation isolation between the inlet pipe 1, the outletpipe 3 and the rotary cooling roller.

The rotary cooling roll outer casing 9 includes an inner spiralstructure prepared from a copper-chromium alloy and having an innerdiameter larger than the outer diameter of the inner heat exchangepassage 8, and the inner heat exchange passage 8 is embedded in therotary cooling roll outer casing 9 to form a spiral water passage. Boththe inner heat exchange passage 8 and the rotary cooling roll outercasing 9 include a hollow structure. The front end cover 7 and the rearend cover 10 are respectively fixed to both sides of the rotary coolingroll outer casing 9 and are perpendicular to the heat conducting sheet8.1. Further, a water inlet hole is provided in the front end cover 7.On the inner heat exchange passage 8, a water outlet hole is provided onthe side close to the rear end cover 10, and a water inlet hole isprovided on the side close to the front end cover 7. The inlet pipe 1and the outlet pipe 3 are provided on the rotary joint 2. Both ends ofthe water inlet sleeve 5 are respectively connected to the water inlethole of rotary joint 2 and the water inlet hole of the inner heatexchange flow passage 8. Both ends of the water outlet sleeve 4 arerespectively connected to the water inlet holes of rotary joint 2 andthe water inlet holes of the front end cover 7. The inner diameter ofthe water outlet sleeve 4 is larger than the outer diameter of the waterinlet sleeve 5. The water outlet sleeve 4 and the front end cover 7 areconnected and fixed by a sealing sleeve 6.

The device of the present disclosure operates in such a manner thatcooling water enters the inner heat exchange passage 8 from the inletpipe 1 via the rotary joint 2 and the water inlet sleeve 5, leaving aplurality of small holes near one end of the rear end cover 10. Afterthe high-pressure water jet is ejected from the small holes, it flowsback along the spiral water passage to the front end cover 7, and flowsout of the water outlet pipe 3 through the water outlet sleeve 4 and therotary joint 2.

When the casting is cooled, the rotating cooling roll outer casing 9 isin direct contact with the high temperature melt to absorb its heat. Theinner spiral structure can increase the mass of the rotating coolingroll outer casing 9, increase the overall heat capacity, and isbeneficial to increase the absorption of the melt heat by the rotatingcooling roll. Further, the contact area of the rotary cooling roll outercasing 9 with the water body is increased, thereby increasing the heatexchange coefficient between the rotary cooling roll and the water body.Since the waterway is a dynamic waterway, turbulence is easily formedinside the water body during the rotation process, which is beneficialto increase the heat exchange coefficient between the rotating coolingroller and the water body, so that the water body quickly absorbs andtransports the heat absorbed by the rotating cooling roller outer casing9, reduces the surface temperature of the cooling roll, facilitates therapid heat exchange of the melt with the cooling water body through thecooling roll as an intermediate medium, so that the melt obtains agreater degree of subcooling.

FIG. 7b is an axial sectional view of the inner wall of the inner heatexchange passage 8 in which a plurality of strip-shaped heat conductingsheet 8.1 parallel to the axial direction are embedded, which furtherincreases the contact area of the cooling water and the solid heatdissipating member. The radial heat transfer of the water inside andoutside the inner heat exchange passage 8 is increased compared with theconventional structure, accordingly increasing the flow rate of theeffective cooling water per unit time. At the same time, when thecooling water enters the internal heat exchange passage 8 from the waterinlet sleeve 5, the water flow is smooth, the turbulence is reduced, andthe smooth flow is ensured through the small hole at the rear end cover10 and contacts with the rotating cooling roller outer casing 9, whichis favorable for increasing cooling capacity of the device and suitablefor large-scale industrial mass production.

The surface of the rotating cooling roll outer casing 9 needs to betreated before the melt is casted. The surface treatment may bemechanical cutting, laser etching, or the like, but is not limited tothese methods. In the embodiment of the present disclosure, 180#˜2000#standard sandpaper can be used for grinding, and different sandpaperscan be used for cross-grinding during grinding. The ten point averageroughness (Rz) of the surface of the rotating cooling roll outer casing9 is controlled to be 1 to 10 μm. An excessive roughness is advantageousfor increasing the heat exchange coefficient, but is also liable tocause heterogeneous nucleation.

During the casting process, the rotation speed is slow, and the spacingof the flaky rare-earth-rich phase will become larger. Fast rotationspeed easily results in chill crystal. When the surface speed of thesurface of the rotary cooling roll is from 1.5 m/s to 2.25 m/s,microstructure of the alloy cast strip can be fine and uniform. At thesame time, the melt casting speed q (casting melt weight/casting time)should be controlled to achieve the best match with the cooling waterflow rate Q. When q/Q is 0.05-0.1, the casting cooling effect can be thebest. For 600 kg melting furnace commonly used in mass production, q/Qcan be, for example, 0.08˜0.09, which can reduce the waterwayconfiguration requirements while satisfying cooling capacity. For asmall 5 to 50 kg induction melting furnace, q/Q can be, for example,0.05 to 0.065, and the equipment has the best cooling capacity. If theq/Q is too large, the loss of the rotating cooling roller is large; ifthe q/Q is too small, the cooling capacity of the device can beimproved. When casting, try to make the melt flow smoothly and spreadevenly onto the surface of the rotating cooling roll.

The disclosure also provides an alloy cast strip for a fine grain rareearth sintered magnet, having R₂Fe₁₄B main phase grains. The alloy caststrip includes R₂Fe₁₄B main phase, as well as the flaky rare-earth-richphase embedded in the grains, the inter-grain rare-earth-rich phase andother unavoidable impurity phases. The main components of the alloy caststrip include the rare earth element R, the additive element T, iron Feand boron B. Wherein R is one or more of La, Ce, Pr, Nd, Sm, Tb, Dy, Ho,Sc, and Y. T is one or more of transition metal elements such as Co, Ni,Cu, Mn, Cr, Ga, V, Ti, Al, Zr, Nb, Mo, and Sn. Among them, the massratio of R in the alloy is 29% to 35%. The mass ratio of T in the alloyis ≤5% or the alloy does not contain the additive element T. The massratio of B in the alloy is 0.85% to 1.1%. If B amount ratio is toolarge, it is prone to form Fe₂B. If the amount ratio of B element is toosmall, it is not conducive to the squareness of the magnet. Theremaining component in the alloy is Fe. During casting cooling, thedifference between average temperature of alloy cast strip on thehighest point of rotary cooling roll and melting point of alloy mainphase ranges from 300˜450° C. In the present disclosure, the main phaseof the alloy is R₂Fe₁₄B main phase. The difference between melting pointof the R₂Fe₁₄B main phase and the temperature of the alloy cast strip isthe degree of subcooling.

The observation of the microstructure of the alloy cast strip of thepresent disclosure involves two modes: (1) magnetic domain microscopy,that is, a polarizing microscope mode; and (2) scanning electronmicroscope backscatter mode. Among them, the contrast of the apolarizing microscope observation photo mainly depends on the crystalplane reflection coefficient and the magnetic moment vector, which canmore clearly observe the microstructure of the crystal grains andmagnetic domains. The contrast of scanning electron microscopebackscatter mode observation photo mainly depends on the alloycomposition, which is used to observe the composition distribution ofthe alloy cast strip. For the alloy cast strip, the grain size is largerthan the magnetic domain, and the large area with different contrast iscaused by different crystal planes of the grain, which is easy toobserve, and the finer contrast is the reflection of the magneticdomain. Compared with different crystal plane contrast differences, themagnetic domain contrast is small, and it is affected by therare-earth-rich phase inside the grain, which is difficult todistinguish in the figure. Therefore, the different contrasts in thefigure correspond to different grains.

Observed by a polarizing microscope, the alloy cast strip provided bythe present disclosure grows along the cross section of the temperaturegradient direction, and no grains are formed from the roll surface tothe free surface. Moreover, the alloy cast strip grains are mainlycharacterized by non-columnar crystals. The grains identified by thedifferent contrasts are no longer elongated columnar crystals grownsubstantially along the temperature gradient direction, but areapproximately equiaxed grains having an aspect ratio of about 1. Here,the definition of the aspect ratio can be seen in FIG. 3. In the sectionalong the thickness direction of the alloy cast strip, the projection ofthe grain profile on the coordinate axis of the normal direction of theroll surface is defined as the longitudinal length 1 of the grain, andthe projection on the coordinate axis of the roll surface is defined asthe lateral width d of the grain. The ratio l/d is the aspect ratio ofthe grain.

In the cross section along the temperature gradient direction, the areaof no less than 60% is covered by crystal grains having an aspect ratioof 0.3 to 2, and the columnar crystal area having an aspect ratio of noless than 3 is no more than 15%. Calculated by numbers, the number ofthe crystal grains having an aspect ratio in the range of 0.3 to 2 is noless than 75%, and the number of columnar crystals having an aspectratio of no less than 3 is no more than 10%, as shown in FIG. 1, whichis characterized with mainly non-columnar crystals. FIG. 2 shows thecolumnar crystal features in the conventional technologies, and thedifferences between the two figures are obvious.

The equivalent circle diameter of the grains is 2.5 to 65 μm in asection along a temperature gradient direction. Wherein, the area of thegrains having an equivalent circle diameter of 10 to 50 μm is not lessthan 80%, and the number of grains having an equivalent circle diameterof 15 to 45 μm is not less than 50%. Among them, the grains in thevicinity of 100 μm of the roll surface are small, and the averageequivalent circle diameter is 6 to 25 μm. The grain size in the vicinityof 100 μm from the free surface is larger, with the average equivalentcircle diameter of 35-50 μm. The equivalent circle diameter of smallnumber of grain scan reach 60-65 μm. Here, the equivalent circlediameter means that the area of the circle having the diameter of theequivalent circle is equal to the grain cross-sectional area. Theaverage equivalent circle diameter is the average value of the grainequivalent circle diameters within a certain area. The equivalent circlediameter of a grain refers to the diameter of a circle having an areaequal to the area of the grain.

Observed by scanning electron microscope backscatter mode, the alloycast strip of the present disclosure has a heterogeneous nucleationcenter in the cross section of the roll surface along the temperaturegradient direction, and the rare-earth-rich phase is radiallydistributed from the center of the heterogeneous nucleus, but the ratiom of such area to the area of the alloy cast strip is not more than 5%.Heterogeneous nucleation centers were not observed in the rest part.That is, there is no visible heterogeneous nucleation center inside thegrain of the alloy cast strip in the area of 95% or more.

The visible heterogeneous nucleation center is the portion which isfirst solidified on the surface of the cooling roll due to the smallnucleation work on the surface of the cooling roll during melt castingcooling. Then, the crystal grains are grown along the temperaturegradient using the portion as a matrix. This is shown in the white arrowmarks in FIGS. 2 and 4.

Observed by scanning electron microscope backscatter mode, there is norare-earth-rich phase or R₂Fe₁₄B main phase grains growing along thecross section of the temperature gradient from roll surface to the freesurface. Moreover, in the range of magnification of 800 to 2000, a clearboundary or partial boundary of the crystal grain can be observed, andthe rare-earth-rich phases identified by the white contrast which aredistributed at the grain boundary and in the grain can be clearlydistinguished. Among them, the geometry of the rare earth phase at thegrain boundary is in an irregular closed state, and the contour is notsmooth. The rare-earth-rich phase in the grain is in the form of flakesor lines, and the profile is smoother than the rare-earth-rich phase atthe grain boundaries.

A section along the temperature gradient direction shows a primarycrystal axis and a secondary crystal axis grown by the primary crystalaxis. Among them, the primary crystal axis boundary is smooth, and theshort axis direction width L₁ is 1.5 to 3.5 μm. The rare-earth-richphase between the secondary crystal axes is in the form of a shortstraight line or a broken dotted line, and the width in the short axisdirection L.₂ is 0.5 to 2 μm. (For the definition of primary crystalaxis and secondary crystal axis of the present disclosure, see Example1)

The rare-earth-rich phase spacing in the alloy cast strip of the presentdisclosure is 0.5 to 3.5 The flaky rare-earth-rich phase appears as aseries of non-strict parallel cluster lines along the temperaturegradient direction (where the non-strict parallel cluster fingers arenot more than 5 degrees), and different non-strict parallel clusterlines can intersect. The measurement process includes: selecting alinear rare-earth-rich phase in a central portion of the non-strictparallel cluster, and making a straight line perpendicular thereto, andthe straight line intersecting the two ends of the non-strict parallelcluster at two points. The distance between the two points measured isD. The number of linear rare-earth-rich phases in the non-strictparallel cluster is n, and the D/(n−1) value is calculated, which is thespacing of the rare-earth-rich phase in the region. For example, fromFIG. 5, D is about 25 and the double-arrow line segment spans elevenlinear rare-earth-rich phases, i.e., n=11, and the space betweenneighboring rare-earth-rich phases is about 2.5 μm.

The present disclosure is described in further detail below by makingreference to Examples and Comparative Examples.

Example 1

Prepare 5 kg of alloy raw material having a composition ofNd_(31.5)Fe_(67.5)B (mass ratio). Before preparation, the raw materialshave been derusted. Melting is carried out using a 5 kg inductionmelting furnace operating at 4 kHz. The metal iron raw material isplaced in the bottom of the corundum crucible, and other metals oralloys other than the Nd alloy are randomly placed in the middle of thecrucible, and the Nd alloy is placed in the upper part of the crucible.Close the induction melting furnace cover, first pump to a low vacuum of5 Pa, and then pump to a high vacuum of 5×10⁻² Pa. After heating for 5minutes with 5 kW power, the power is increased to 8 kW and heat for 3minutes, and then power is further increased 10 kW and heat for 2minutes. At this time, the bottom raw material of the crucible is redand at a high temperature. Then, the power is reduced to 4 kW, and thevacuum valve is closed, and argon gas having a purity of 99.99% isintroduced until the pressure reaches 50 kPa. After one minute, open thevacuum valve and pump again to 2×10⁻² Pa, then close the vacuum valveand refill with argon to 40 kPa. Increase the power to 15 kW and heatthe alloy until it begins to melt, and the melt surface temperature is1150° C. After heating for 2 minutes, the power is decreased to 12 kWand maintained for 2 minutes and then increased to 18 kW. When thetemperature reaches 1230° C., power is decreased to 3 kW, and the melttemperature drops to 1190° C. Then increase the power to 20 kW. Repeatthe above processes to control the melt surface temperature at 1300° C.until raw material melts completely. Then increase the power to 25 kWand start refining until the melt surface temperature rises to 1400° C.,and then reduce the power to 16 kW. A small amount of dross in the meltadheres to the crucible wall under strong electromagnetic stirring. Whenthe melt temperature is stable at 1480° C., the power is approximately13 kW, and the melt state is stable at this time, and the apparent stateis relatively clear.

The Rz of the rotary cooling roll outer cover surface is 1 and thesurface linear velocity is 2.25 m/s. The melt casting speed q is 0.1kg/s. Cooling water flow Q is 7 m³/h, e.g. 1.95 kg/s. Then q/Q=0.05.Alloy cast strip is obtained through casting cooling. The surfacetemperature of the alloy cast strip is measured to obtain a degree ofsubcooling of 450° C. when the melt is solidified. During the castingprocess, as the melt in the crucible is reduced, the heating power isappropriately reduced. After the casting is completed, it is cooled in awater-cooled turntable for 1 hour, and the alloy cast strip is takenout. Fifty alloy cast strips are randomly taken to measure thethickness, which is 0.2 to 0.58 mm.

FIG. 1 and FIG. 9(a) are photomicrographs of the alloy cast strip undera polarizing microscope. It presents a number of different contrastareas, corresponding to different crystal planes. By performing a manualstroke on FIG. 9(a), the morphology of each grain in the alloy caststrip can be discerned as shown in FIG. 9(b). FIG. 9(b) is binarized toobtain FIG. 9(c). Then use the image processing software to remove theincomplete grain portion of the boundary, and count the area of allremaining grains (shown in the shaded part of FIG. 9(d)) and thereciprocal of the aspect ratio of the grain. The particle aspect ratiol/d and the equivalent circle diameter r are shown in Table 1. The grainnumbers in Table 1 correspond one-to-one with the grain numbers in theshaded area in FIG. 9(d).

TABLE 1 Aspect ratios and equivalent circle diameters of the alloy castcrystal grains shown in FIG. 9(a) Equivalent circle Grain number Aspectratio l/d diameter r/μm 1 0.421 7.719 2 0.308 6.236 3 0.759 15.802 41.400 21.683 5 1.256 23.309 6 0.459 13.145 7 0.368 27.883 8 1.409 39.9089 0.692 21.764 10 1.744 60.415 11 1.394 6.025 12 0.850 41.647 13 1.00028.347 14 1.091 4.575 15 1.400 38.746 16 1.520 25.595 17 0.761 20.172 180.705 24.401 19 1.769 28.187 20 0.825 13.421 21 0.979 8.081 22 0.81447.701 23 1.756 11.252 24 1.161 37.876 25 1.036 22.336 26 1.335 43.50327 0.889 12.036 28 0.447 10.281 29 1.008 22.627 30 1.370 21.032 31 1.10322.979 32 0.733 16.915 33 0.447 4.347 34 1.522 51.282 35 0.794 10.005 361.108 23.251 37 0.714 41.038 38 0.745 23.051 39 1.359 12.950 40 2.4445.686 41 1.485 18.881 42 1.198 19.734 43 2.909 3.186 44 1.409 12.974 451.261 6.225 46 0.580 11.364 47 1.629 22.483 48 1.682 24.163 49 0.72627.148 51 1.783 33.562 52 1.235 3.288 53 0.482 19.776 54 2.049 12.600 550.537 21.124 56 0.530 22.772 57 0.917 11.134 58 1.216 27.088 59 0.49320.779 60 1.011 17.087 61 1.070 24.865 62 0.739 18.855 63 1.266 42.63165 1.271 11.994 66 0.798 17.071 67 1.159 8.682 68 0.870 26.510 69 1.61833.974 70 0.986 15.360 71 0.956 22.530 72 1.643 10.026 73 1.386 26.45974 1.568 30.974 75 0.850 19.887 76 1.171 27.181 77 1.383 14.822 78 0.69616.868 79 1.034 20.330 80 1.389 40.311 81 1.500 6.363 82 1.355 13.435 830.918 27.001 84 0.975 7.560 85 0.726 10.875 86 0.553 13.448 87 1.79117.842 88 1.260 18.497 89 1.277 21.902 90 1.614 31.257 91 1.380 20.07292 0.880 13.389 93 0.862 33.393 94 1.051 7.788 95 0.523 5.485 96 0.8644.411 97 0.984 12.505 98 0.636 5.495 99 0.622 5.818 100 1.316 3.074 1011.483 5.821 102 2.912 10.055 103 0.886 14.603 104 1.164 40.106 105 1.04630.185 106 0.910 11.259 107 0.656 11.767 108 1.587 47.192 109 0.67611.114 110 1.076 58.392 111 2.465 22.894 112 0.755 9.215 114 1.02033.117 115 1.075 9.370 116 2.814 29.253 117 0.664 16.717 118 0.74327.724 119 0.839 19.920 120 0.522 10.566 121 1.370 11.620 122 2.00011.381 123 0.636 11.751 124 2.000 4.277 125 0.907 9.976 126 1.739 4.829127 1.350 4.352 128 0.902 11.947 129 1.219 17.874 130 0.564 13.662 1310.908 19.678 132 0.641 23.265 133 0.641 17.620 134 0.687 11.421 1350.475 21.201 136 1.054 11.958 137 0.556 9.254

From Table 1, l/d in this partial region is 0.3 to 3, in which the arearatio of grains having l/d of 0.3 to 2 is about 98%, the number ratio ofsuch grains is 96.3%, and there is no grain having an aspect ratiogreater than or equal to 3. The grain having the largest area is grainNo. 10, which has a radius r of about 60 The grain having the smallestarea is grain No. 100, which has a radius r of about 3.074 For grainswith r of 10 to 50 μm, their area ratio is about 82.3%, and the numberratio of grains having r of 10 to 45 μm is about 51.2%. Overall, thegrains near the side of the roll surface are small, and the ones nearthe side of the free surface is large. In the range of 100 μm from theside of the roll surface, the average equivalent circle diameter of thegrains is about 6 to 15 μm, and the average equivalent circle diameterof the grains is from 25 to 40 μm in the range of 100 μm from the freesurface side. It is worth noting that in FIG. 1 and FIG. 9(a), there arelarge abnormal grains near the side of the roll surface. On the onehand, it may be because the orientation of some grains is affected bythe cooling roll surface, and the grain orientation degree is relativelyhigher than the side of the free surface, so that it is difficult todistinguish the grain boundaries; on the other hand, the cooling processmay not be fast enough, resulting in some small grains recrystallizingto form larger grains.

Note: Due to the influence of the Nd-rich phase inside the alloy caststrip, it is difficult for the computer to automatically identify thegrain boundaries according to different contrasts. Manual stroke may bean accurate way to distinguish such alloy cast strips. Although theremay be some errors, the measurement data will not affect thecorresponding statistical regularity of the test quantity because of thestatistics of a large number of grains. For the range of grain sizes,the error caused by the measurement is negligible.

FIG. 10(a) is an overall photograph of the cross section of the alloycast strip in the temperature gradient direction of the presentembodiment, the magnification is 600 times, the upper portion is a freesurface, and the lower portion is a roller surface. It can be seen fromFIG. 10(a) that along the temperature gradient section, there is noheterogeneous nucleation center as indicated by the white arrows in FIG.2 and FIG. 4, and the flaky Nd-rich phase is randomly distributed in thedirection of the long axis, not in radial shape along the temperaturegradient direction. No flaky grains were observed to grow from the rollsurface to the free surface. FIG. 10(b) is a photograph when the whiterectangular frame area in FIG. 10(a) is enlarged to 2000 times. As canbe seen from FIG. 9, the Nd-rich phase of the grain boundary is in anirregular closed state, and a flaky or linear Nd-rich phase inside thegrain is embedded in the grain. This is further confirmed by polarizedmicroscope photo and scanning electron microscope backscatter photo inthe subsequent examples.

As can be seen from FIG. 10(b), the grain size in this region is 20 to25 μm. The Nd-rich phase spacing is 0.6 to 2.7 μm. The flaky grains havetwo states, some of which are coarser, as shown by the white arrow inFIG. 10(b), and the Nd-rich phase spacing is about 1.5 to 2.7 μm. Theseflaky main phase grains are the portions which are preferentiallysolidified. More flaky grains are relatively small, and the Nd-richphases are spaced apart by about 0.5 to 1.8 μm, some of which areproduced by the coarser flaky main phase grains on the sideperpendicular to the long axis. There are relatively coarse plate-likecrystal regions and a finer plate-like crystal regions. In the presentdisclosure, coarser flaky grains are a primary crystal axis and finerflaky grains are a secondary crystal axis. In the scanning electronmicroscope backscatter mode, the Nd-rich phase of the primary crystalaxis is smooth and bright, and the contrast of the secondary crystalaxis is slightly dark, showing in form of a short straight line or abroken line. In the rapid non-equilibrium solidification processprovided by the present disclosure, the high temperature melt undergoesa greater degree of subcooling and reaches near the ternary eutectictemperature of the alloy in a short time (corresponding to E₂ in theternary liquid phase projection of NdFeB, where the main phase T1, theboron-rich phase T2, and the Nd-rich phase are simultaneouslyprecipitated from the liquid phase at this point). Under this extremecondition, the tendency of the main phase grains and the Nd-rich phasealong the temperature gradient are weakened by effect of the specificmelt state, greater supercooling degree and temperature gradient, andeutectic or eutectoid growth is dominant and form Feature morphology.The spacing of Nd-rich phases of alloy cast strip is smaller and thedifference between the roll surface and the free surface is smaller thanin the conventional technologies.

In FIG. 9 and FIG. 10, the alloy cast strip grains of the presentdisclosure are mainly non-columnar grains, and most of them arehomogeneous nucleation of the melt, and l/d is 0.3-2, and no growth ofl/d>3 main phase grains along the temperature gradient is observed. TheNd-rich phase spacing is smaller and is more suitable for preparing finegrain rare earth sintered magnets.

Select the same batch of 5 alloy cast strips for calculation, and findthe average value. The relevant parameters are listed in Table 2. Thedifference of maximum thickness and minimum thickness of the alloy caststrips used for the measurement is at least 0.2 mm.

The alloy cast strip is crushed sequentially by hydrogen crushing andjet mill to prepare powders, and the powders are press formed, sintered,and the like to form magnets. After the jet milling, the particle sizeof the powders is measured using a laser particle size analyzer. Afterheat treatment, three sintered samples were randomly selected, and therare earth components of the sintered samples were tested by inductiveplasma atomic emission spectrometry (ICP-AES), and the performanceparameters of the magnets were measured. The specific values are shownin Table 3.

Comparative Example 1

Prepare 5 kg of alloy raw material having a composition ofNd_(31.5)Fe_(67.5)B (mass ratio) and the alloy raw materials beforepreparation are subjected to rust removal treatment. Melting is carriedout using a 5 kg induction melting furnace operating at 4 kHz. The metaliron raw material is placed in the bottom of the corundum crucible, andother alloys except the Nd alloy are randomly placed in the middle ofthe crucible, and the Nd alloy is placed on the upper part of thecrucible. Close the induction melting furnace cover, pump to a lowvacuum of 5 Pa, then pump to a high vacuum of 2×10⁻² Pa. After heatingfor 5 minutes with 5 kW power, the power is increased to 8 kW and heatfor 3 minutes, and power is further increased to 10 kW and heat for 2minutes. The raw material at the bottom of the crucible has been red andat a high temperature. The vacuum valve is closed and charged with argongas to 40 kPa, and then the power is increased to 15 kW to continueheating, and after 2 minutes, the power is again raised to 25 kW. Theraw materials in the refining process are completely melted and thetemperature is finally stabilized at 1400° C. when the melt is casted,and the casting speed q is 0.1 kg/s. Cool down with a conventionalcooling roll without internal thread structure, and the cooling waterflow Q of the rotary cooling roller is 7 m³/h, which is 1.95 kg/s. Andq/Q=0.05, the same as Example 1. Using the same estimation method as inExample 1, the degree of subcooling during melt solidification was about298° C. Finally, an alloy cast strip having an average thickness of 0.3mm is obtained. Remaining preparation process and measurement methodsare the same as in Example 1.

FIG. 11(a) is a polarizing microscope photograph of the microstructureof the alloy cast strip of Comparative Example 1. FIGS. 11(b), 11(c),and 11(d) show the same grain measurement method as that of FIG. 9, andspecific data of the grain aspect ratio and the equivalent circlediameter are shown in Table 4. It can be seen from the figure that thealloy cast strips are mainly in columnar shape along the cross sectionof the temperature gradient direction, and the columnar grains growradially from heterogeneous nucleation of the roll surface toward thefree surface. It is estimated that the area ratio of grains having anl/d of 0.3 to 2 is only about 15%, and the number ratio of such grainsis only 44%. The area ratio of grains with r of 10 to 50 μm is 31%, andmore grains have r>50 That is, the average grain size thereof is largerthan that in Example 1.

FIG. 12 is a scanning electron microscope backscattered photograph of analloy cast strip. It can be seen from the figure that the white Nd-richphase is radially distributed along the direction of the temperaturegradient from the center of the heterogeneous nucleation, with spacingof about 3 to 10 μm. The grain boundary and the Nd-rich phase cannot berecognized in this figure, and its distribution characteristics areobviously different from those shown in FIG. 10 in the Example 1. Thewhite Nd-rich phase distribution is affected by the temperaturegradient, and the grain boundary and the interior distribution of therare-earth-rich phase along the temperature gradient are dominant. Therich phase distribution in other directions is less. The rare-earth-richphase at the grain boundary does not show a closed distribution. In FIG.12, there are many lateral (approximately perpendicular to thetemperature gradient direction) and shorter flaky grains among the mainphase grains radially growing from the surface of the roll to the freesurface, which is defined as a secondary crystal axis in the presentdisclosure. However, the morphology is different from that in Example 1.

Another five alloy cast strips with different thickness are selected fortesting. The test results can be seen in Tables 2 and 3.

TABLE 2 Structural parameters of alloy cast strips in Example 1 andComparative Example 1 l/dϵ [0.3, 2] grain l/d > 3 rϵ [10, 50] grain rϵ[15, 45] grain ratio Grain % area ratio number ratio m L₁(μm) L₂(μm)Example 1 Area: 97% Area: 0% 80% 50%   0% 1.5-2.7: 0.5-1.8: Number:96.3% Number: 0% Comparative Area: 15% Area: 52% 32% 34% 97.5%  3-9.5:1.5-2.3: Example 1 Number: 40% Number: 35%

Where m is the area ratio of the rare-earth-rich phase having a radialpattern.

TABLE 3 Test data of particle size and magnet properties of the powdersprepared in Example 1 and Comparative Example 1 Prepare powder by jetmill TRE D₁₀ D₅₀ D₉₀ (wt. %) B_(r)(kGs) H_(cJ) (kOe) (BH)_(Max)(MGOe)Example 1 1.36 3.45 6.57 30.7 13.29 9.63 42.41 Example 1 30.4 13.29 9.5842.33 Example 1 30.6 13.28 9.60 42.38 Comparative Example 1 1.55 3.947.58 30.2 13.29 9.53 42.23 Comparative Example 1 30.3 13.26 9.52 42.09Comparative Example 1 30.0 13.31 9.50 42.13Where TRE (wt. %) is the total rare earth weight percentage, Br, Ha and(BH)_(Max) are respectively remanence, coercivity and maximum energyproduct of the magnet at room temperature.

As can be seen from the data in Table 3, the powders prepared from thealloy cast strip of Example 1 have a smaller particle size, D₉₀/D₁₀. Itis relatively small, namely more uniform and fine, which is favorablefor grain refinement of sintered magnets. In the prepared sinteredmagnet, the rare earth content TRE is about 0.3% by weight higher thanthat of Comparative Example 1, and the coercive force Ha and maximummagnetic energy product (BH)_(Max) are relatively high, with nosignificant change in remanence Br. The overall performance of themagnet is improved. When jet mill powder particle size D₅₀ is close toor smaller than the spacing of the Nd-rich phases, the rare earthutilization rate is obviously improved, and the improvement of thecoercivity of the magnet prepared by alloy casting strip with the sameformula will be more obvious.

TABLE 4 Aspect ratio and equivalent circle diameter of the alloy caststrip crystal grains shown in FIG. 11(a) Equivalent circle Grain numberAspect ratio l/d diameter r/μm 1 0.321 13.903 2 1.750 23.863 3 0.51223.947 4 0.780 6.346 5 0.843 21.887 6 0.818 24.190 7 0.500 10.656 81.969 55.660 9 0.858 17.655 10 0.444 6.711 11 2.861 23.971 12 4.01249.714 13 3.425 13.756 15 2.292 20.556 16 5.993 62.650 17 2.407 106.04518 5.324 51.783 19 3.786 6.103 22 0.896 13.546 23 5.050 34.750 24 2.49234.193 25 2.978 11.490 26 2.097 28.689 27 2.879 11.942 29 4.708 8.316 301.226 11.110 31 1.050 3.710 32 1.185 6.887 33 4.000 5.015 39 2.08032.655 40 3.778 8.098 41 1.883 21.648 42 3.518 68.843 43 2.102 30.214 444.02 65.204 45 1.59 9.458 46 4.95 52.130 50 6.00 11.669 51 3.33 5.747 532.67 10.980 54 2.46 21.973 55 8.57 13.234 56 3.56 46.702 57 3.66 52.51158 1.58 7.533 60 1.76 16.701 61 2.52 28.817 64 3.78 8.663 65 1.08 14.82866 0.68 12.094

Example 2

Prepare alloy raw material 600 kg with components ofNd_(24.4)Pr_(6.1)DyCoCu_(0.1)Al_(0.65)Ga_(0.1)B_(0.97)Fe_(ball) (massratio). It is smelted in a 600 kg induction melting furnace. The mainsteps are similar to those of Example 1, but the corresponding poweradjustment range is larger. When the impurity gas in the alloy isexcluded, the power fluctuates between 120 kW and 240 kW, and then theargon gas having a purity of 99.99% is introduced to increase thepressure to 40 kPa. Vacuum again to 2.2×10⁻² Pa, refill with argon to 40kPa. The power is increased for melting, and the power varies from 380kW to 520 kW. After cyclical overheat treatment, the raw material iscompletely melted before the melt is heated to 1300° C. Use a rotarycooling roll as shown in FIG. 7a , and the temperature at the time ofcooling casting is 1400° C. The melt casting speed q is controlled to be0.8 kg/s. Cooling water flow Q is 40 m³/h, which is 11.11 kg/s. Thenq/Q=0.07. The surface of the rotary cooling roll is Rz=8.6 μm, and thesurface linear speed of the cooling roll surface during the casting is1.5 m/s. An alloy cast strip having a thickness of 0.12 to 0.48 mm isprepared. The melt solidification process has a degree of subcooling ofup to 365° C.

As can be seen from FIG. 13 and FIG. 14a , the grain size of the alloycast strip of Example 2 is relatively uniform and fine, r isapproximately distributed in the range of 3 to 60 μm, but l/d isrelatively large, namely 0.3 to 4. The rare-earth-rich phasedistribution is non-radial, with a spacing of about 0.8 to 2.8 μm, andlarger in some regions. The heterogeneous nucleation center is visiblein the lower right corner of FIG. 14a . However, the rare-earth-richphase did not exhibit a through-radial growth and soon terminated atabout 70 μm from the surface of the roll. Based on the area shown inFIG. 14a , the area ratio is about 5%. At the same time, thedistribution of some grain boundaries and the rare-earth-rich phaseinside the grain can be clearly observed. FIG. 14b is a partialphotograph of the central portion near the surface of the roll surfaceof FIG. 14a magnified 4000 times. The primary crystal axis is located inthe middle of the grains, and the secondary crystal axis is grownperpendicular to the axial direction of the primary axis. Comparing FIG.13 with FIG. 14a , it can be seen that the rare-earth-rich phase of thegrain boundary is in an irregular closed state, and the rare-earth-richphase in the grain is relatively regular, and is in a smooth line orintermittent short-line state, and the spacing is about 0.5-1.8 μm. Fivealloy strips with different thicknesses were selected and theircharacteristic parameters are listed in Table 5. The maximum thicknessand minimum thickness of the selected alloy strips differed by at least0.2 mm.

Example 3

The alloy composition isNd_(26.3)Pr_(8.6)Ga_(0.56)Al_(0.19)Cu_(0.1)Zr_(0.19)B_(0.89)Fe_(ball),casting temperature is 1500° C., Rz=10 μm, surface linear velocity is 2m/s, melt casting speed q is 1 kg/s, and cooling water flow Q is 36m³/h, that is, Q is 10 kg/s, q/Q=0.1. The rest is the same with Example2. The degree of subcooling during melt solidification is 300° C., andthe characteristics of the alloy cast strips are shown in FIG. 15 andFIG. 16. The alloy cast strip test data is shown in Tables 5 and 6.

FIG. 15 and FIG. 16 show in situ observations to further verify thestructural characteristics of the aforementioned alloy cast strip. Thespecific form of the alloy cast strip of Example 3 is more similar tothat of Example 2, and is affected by the temperature greater than thatof Example 1. At 800× magnification, the backscattering mode of thescanning electron microscope is used to observe that the grainboundaries near the free surface are more clear, while the roll surfaceis basically unable to distinguish the grain boundaries. The moredetailed internal structure is similar to that of Example 2 and will notbe repeated here.

Table 7 is the grain aspect ratio and equivalent circle diameter dataobtained after performing identification process of the alloy cast stripin Example 3 (FIG. 16) as the same with that in FIG. 9 (FIG. 19).

Comparative Example 2 and Comparative Example 3

The formulation components and the casting process of ComparativeExample 2 and Comparative Example 3 were the same as those of Example 2and Example 3, respectively, wherein the casting temperature ofComparative Example 2 is 1380° C. Cooling is carried out using therotary cooling roll of the present disclosure. In Comparative Example 3,casting temperature is 1492° C. It is cooled by a conventional rotarycooling roller. Further, in the smelting processes of ComparativeExample 2 and Comparative Example 3, the cyclic overheat treatment isnot performed, and the melt temperature gradually increased from low tohigh during the smelting process. During the casting process, the melthas a degree of subcooling of 200 to 300° C. Among them, the meltsupercooling degree in the casting process of Comparative Example 2 is300° C., which is higher than the subcooling degree of the melt of 245°C. in Comparative Example 3, indicating that the cooling capacity of therotary cooling roll shown in FIG. 7a is larger than that of theconventional cooling roll. However, compared to Example 2, it is lowerthan the subcooling degree of 365° C. in Example 2, which may be due tothe fact that the melt of Example 2 is subjected to the cyclic overheattreatment, resulting in the melt being able to withstand a greaterdegree of subcooling. Since the melt once solidified, the heat exchangeefficiency of the solid alloy to the surface of the cooling roll will belower than the heat exchange efficiency between the melt and the coolingroll, resulting in a high surface temperature of the solid alloy caststrip. The microstructure of the alloy cast strip is similar to that ofComparative Example 1, and there is no essential difference, and therare-earth-rich phase is radial, as shown in FIGS. 17 and 18. Thepolarized photomicrograph shows a grain morphology very similar to thatof FIG. 2, consistent with the conventional alloy cast structure strip.The properties of the alloy cast strips prepared and the sinteredmagnets finally prepared in Comparative Example 2 and ComparativeExample 3 can be seen in Tables 5 and 6.

TABLE 5 Comparison of alloy cast strips in Example 2 and 3 with those inComparative Examples 2 and 3 l/dϵ [0.3, 2] l/d > 3 rϵ [10, 50] grain rϵ[15, 45] grain grain ratio Grain % area ratio number ratio m L₁(μm)L₂(μm) Example 2 Area: 63% Area: 12% 87.5% 71% 0.2% 1.5-2.8: 0.8-1.8:Number: 77.5% Number: 7.6% Comparative Area: 11% Area: 63%  29% 33%100%  2.5-8:   1-2.2: Example 2 Number: 21.5% Number: 43% Example 3Area: 60% Area: 11.5% 89.2% 77%  5%  2-3.5: 0.8-2:  Number: 75% Number:8% Comparative Area: 20.3% Area: 56% 37.5% 34%  98%  3-12: 1.8-2.6:Example 3 Number: 29% Number: 39%

TABLE 6 Test data of particle size and magnet properties of the powdersprepared in Examples 2 and 3 and Comparative Examples 2 and 3. Preparepowder by jet mill D₁₀ D₅₀ D₉₀ TRE (wt. %) B_(r)(kGs) H_(cJ) (kOe)(BH)_(Max)(MGOe) Example 2 1.53 4.41 8.21 30.0 13.05 18.75 42.25 Example2 30.0 13.03 18.71 42.23 Example 2 29.9 13.3 18.72 42.77 ComparativeExample 2 1.61 4.65 8.44 29.9 13.02 18.69 42.18 Comparative Example 229.9 13.01 18.73 42.19 Comparative Example 2 29.7 13.05 18.65 42.23Example 3 1.58 4.51 8.33 34.2 12.93 21.19 41.80 Example 3 33.9 12.9520.37 41.86 Example 3 33.9 12.96 21.90 41.86 Comparative Example 3 1.624.71 8.51 33.6 12.97 21.07 41.79 Comparative Example 3 33.6 12.99 20.7941.83 Comparative Example 3 32.9 12.93 20.86 41.76

TABLE 7 Aspect ratio and equivalent circle diameter of the alloy caststrip shown in FIG. 16 Equivalent circle Grain number Aspect ratio l/ddiameter r/μm 1 2.239 50.291 2 0.750 6.705 3 0.676 30.494 4 1.526 25.9565 2.119 30.772 6 1.743 60.290 7 2.471 33.636 8 3.181 50.641 9 3.10832.936 10 1.917 38.297 12 2.793 39.613 13 1.315 42.652 14 2.434 38.41915 1.253 19.829 16 2.323 33.443 17 3.652 27.665 18 2.442 35.342 19 1.78431.890 21 2.605 11.497 22 2.575 31.446 23 1.387 24.386 24 3.246 14.82525 3.091 45.456 26 1.175 48.544 27 1.537 22.038 28 3.483 48.936 29 1.56829.639 30 0.545 34.729 31 1.953 34.250 32 1.144 27.165 33 1.667 39.85734 1.562 35.201 35 1.106 33.935 36 1.272 29.640 37 1.550 26.747 38 2.00640.601 39 1.032 10.834 40 1.727 26.744 41 1.580 29.005 42 1.884 33.25443 1.053 34.506 46 1.000 31.515 47 1.409 11.266 49 1.533 22.050 50 2.23819.288 51 1.537 35.051 52 1.010 22.668 53 1.673 19.901 54 1.800 17.29855 1.338 16.756 56 1.30 26.115 57 1.97 31.451 59 2.44 14.505 60 2.2121.891 61 1.21 17.501 62 1.68 18.635 63 0.85 15.366 64 0.98 8.093 651.21 18.375 66 1.39 13.458 67 0.83 12.869 68 0.91 27.153 69 0.56 8.91670 0.52 26.448 71 0.97 18.995 72 0.70 21.637 73 0.95 12.927 74 0.6919.953 75 1.36 5.155 76 1.00 9.974 77 1.03 13.207 78 0.96 12.435 80 0.5613.458

Example 4-6 and Comparative Example 4-6

In Examples 4-6 and Comparative Examples 4-6, alloy cast strips withplural formulation were prepared using a 5 kg induction melting furnace.In the preparation process, Examples 4-6 are similar to Example 1 exceptfor the casting temperature, and Comparative Examples 4-6 were similarto Comparative Example 1, and the microstructure of the alloy castsheets is similar to that of Example 1 and Comparative Example 1,respectively. The specific alloy formula is as follows:

The alloy formulation of Example 4 and Comparative Example 4 isNd._(20.88)Pr_(6.5)Dy_(5.68)Co_(0.92)Cu_(0.13)Ga_(0.5)Al_(0.22)B_(0.85)Fe_(Ball).The casting temperature is 1400° C. The alloy formulation of Example 5and Comparative Example 5 is Nd_(0.29)Fe₇₀B, the casting temperatureswere 1450° C. and 1285° C., respectively. The alloy formulation ofExample 6 and Comparative Example 6 isNd._(25.3)Pr_(4.9)B_(1.1)Co_(0.32)Nb_(0.12)Al_(0.13)Cu_(0.18)Ga_(0.14)Fe_(Ball).The casting temperature is 1400° C.

The obtained alloy cast strip is subjected to the same powdering andheat treatment process as in Example 1 to prepare a magnet. The totalmass of the rare earth in the magnet obtained from the alloy cast stripof Example 4-6 is usually 0.1% to 0.3% more than that of thecorresponding comparative example, and the coercive force is high, asshown in Table 8.

TABLE 8 Test data of particle size and magnet properties of the powdersprepared in Examples 4-6 and Comparative Examples 4-6 Prepare powder byjet mill D₁₀ D₅₀ D₉₀ TRE (wt. %) B_(r)(kGs) H_(cJ) (kOe)(BH)_(Max)(MGOe) Example 4 1.50 4.42 8.23 32.0 12.01 31.20 38.90 Example4 32.2 11.93 32.14 38.68 Example 4 32.1 12.00 31.28 38.87 ComparativeExample 4 1.57 4.61 8.39 29.9 12.07 30.54 39.04 Comparative Example 429.9 12.10 30.23 39.20 Comparative Example 4 29.7 12.11 30.03 39.23Example 5 1.51 4.41 8.21 28.01 14.58 8.17 48.03 Example 5 27.99 14.578.13 47.95 Example 5 28.00 14.59 8.21 48.05 Comparative Example 5 1.624.72 8.49 27.99 14.53 7.95 47.95 Comparative Example 5 27.97 14.58 7.8148.09 Comparative Example 5 27.97 14.58 7.79 48.13 Example 6 1.48 4.398.19 29.0 13.98 11.32 45.99 Example 6 28.8 13.98 11.28 45.97 Example 629.0 13.97 11.35 45.93 Comparative Example 6 1.58 4.61 8.35 28.6 13.9811.23 45.98 Comparative Example 6 28.6 13.97 11.16 45.98 ComparativeExample 6 28.5 14.00 11.13 45.95

In order to compare the present disclosure with the conventional alloycast strip more clearly and concisely, in the present disclosure, thedata of Table 1, Table 4 and Table 7 are selected as representative, andthe feature comparison data is obtained after conversion, as shown inFIG. 20 and FIG. 21.

As shown in FIG. 20, in the embodiment, l/d is mainly concentrated in0.3 to 2, and the number of more than 3 is extremely few. In thecomparative example, the aspect ratio of the crystal grains is 0.3 to 6,and some is up to 8, and the distribution is relatively dispersed.Further, in the examples, r is mostly concentrated in 6 to 45, and inthe comparative example, r is mostly 2 to 25. The r of a few largegrains can reach more than 100 μm. That is, in the examples, finecrystal grains and large crystal grains are relatively less incomparison, and l/d is concentrated in the vicinity of 1. It is shownthat the grains are more uniform in the examples, and the medium-sizedequiaxed grains are mostly.

FIG. 21 (a) shows the cumulative distribution of grain area with l/d.From the figure, the rise trend when the example curve is at l/d<2 issignificantly larger than that of the comparative example. That is, themedium-axis crystal in the example occupies are dominant, and the grainsof l/d>4 are extremely few. In the comparative example, the rise is slowwhen l/d<2. That is, the columnar grain is a main grain form in thecomparative example. FIG. 21(b) shows the cumulative distribution ofgrain area with r. The curve of the comparative example has a slowrising trend, and the grain r is distributed at 40 to 100 μm. In theembodiment, the grains r rise steeply in the range of 15 to 50 μm, thatis, a large number of grains are concentrated in this range. ComparingFIG. 20 with FIG. 21, it is understood that the medium-axis crystal ofthe alloy cast strip of the example is a main crystal form, and theaverage grain size is finer and uniform than the comparative example,and the grain size is medium. This microstructural feature is derivedfrom the higher nucleation rate caused by the higher degree ofsupercooling in the examples, and also determines the smaller spacing ofthe rare-earth-rich phase inside the grain. From this point of view, therefinement of the rare-earth-rich phase inevitably brings about thegrain refinement.

Compared to conventional alloy casts, when the jet mill powder size D₅₀is closer to or slightly larger than spacing of the rare-earth-richphase, the final magnet grain size will be smaller. The advantageous theperformance of the magnet prepared by the alloy cast sheet in thepresent disclosure will be more obvious. However, the magnets preparedin the examples of the present disclosure are limited by the jet millingand sintering process, and the average grain size of the powder and thefinal magnet is large, and the performance of the magnet is slightlyimproved even under such conditions. It is foreseen that the improvementof the performance of the final magnet of the alloy cast strip by thepresent disclosure will be more apparent with the optimization of thefinal sintering magnet grain refining process, and is not limited to theimprovement effect in the embodiment of the present disclosure.

It should be noted that the above-described embodiments are merelyillustrative of the disclosure and are not intended to limit thedisclosure. Other variations or modifications may be made by thoseskilled in the art in light of the above description. There is no needand no way to exhaust all of the implementations. Obvious changes orvariations resulting therefrom are still within the scope of thedisclosure.

What is claimed is:
 1. An alloy cast strip comprising grains havingR₂Fe₁₄B-type compound as main phase, wherein: R denotes a rare earthelement, Fe denotes iron, and B denotes boron; the grains includenon-columnar grains having an aspect ratio in a range from 0.3 to 2 andcolumnar grains having an aspect ratio equal to or larger than 3; aratio of an area of the non-columnar grains to a total area of thegrains is equal to or larger than 60% and a ratio of a number of thenon-columnar grains to a total number of the grains is equal to orlarger than 75%; and a ratio of an area of the columnar grains to thetotal area of the grains is equal to or smaller than 15% and a ratio ofa number of the columnar grains to the total number of the grains isequal to or smaller than 10%.
 2. The alloy cast strip according to claim1, wherein one of the grains includes: an R₂Fe₁₄B-type main phase; andrare-earth-rich phases including: in-grain rare-earth-rich phasesembedded in the one of the grains, a spacing between neighboring ones ofthe in-grain rare-earth-rich phases is in the range of 0.5-3.5 μm; andboundary rare-earth-rich phases distributed at a boundary of the one ofthe grains.
 3. The alloy cast strip according to claim 2, wherein therare-earth-rich phases do not extend from a first surface of the alloycast strip to a second surface of the alloy cast strip, the firstsurface and the second surface being opposite to each other.
 4. Thealloy cast strip according to claim 2, wherein the boundaryrare-earth-rich phases are distributed in an irregularly closedconfiguration along a temperature gradient direction cross section. 5.The alloy cast strip according to claim 1, further comprising: anadditive element T, the additive element T including at least one of Co,Ni, Cu, Mn, Cr, Ga, V, Ti, Al, Zr, Nb, or Sn; wherein R includes atleast one of La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Sc, or Y.
 6. The alloy caststrip according to claim 5, wherein a mass ratio of B in the alloy caststrip is 0.85% to 1.1%.
 7. The alloy cast strip according to claim 1,wherein equivalent circle diameters of the grains are in a range from2.5 to 65 μm in a cross section along a temperature gradient direction.8. The alloy cast strip according to claim 7, wherein a ratio of an areaof one or more of the grains that have an equivalent circle diameter of10 to 50 μm to a total area of the grains is equal to or larger than80%.
 9. The alloy cast strip according to claim 8, wherein a ratio of anarea of one or more of the grains that have an equivalent circlediameter of 15 to 45 μm to the total area of the grains is equal to orlarger than 50%.
 10. The alloy cast strip according to claim 7, wherein,in the cross section along the temperature gradient direction, anaverage equivalent circle diameter of one or more of the grains that arein a range of 100 μm to a first surface of the alloy cast strip is in arange from 6 to 25 μm, and an average equivalent circle diameter of oneor more of the grains in a range of 100 μm to a second surface of thealloy cast strip is in a range from 35 to 65 μm, the first surface andthe second surface being opposite to each other.
 11. The alloy caststrip according to claim 1, wherein a ratio of an area of one or more ofthe grains that have a heterogeneous nucleation center to a total areaof the alloy cast strip is equal to or smaller than 5%.
 12. The alloycast strip according to claim 1, wherein the grains do not extend from afirst surface of the alloy cast strip to a second surface of the alloycast strip, the first surface and the second surface being opposite toeach other.